1. Field of the Invention
This invention pertains to an improved method of chemically cleaning sulfide-containing scale from metal surfaces. The novel process utilizes aqueous acid cleaning solutions containing an aldehyde in amounts sufficient to prevent or substantially prevent the evolution of hydrogen sulfide gas, and the aldehyde is generated in situ.
2. Description of the Prior Art
Many sources of crude oil and natural gas contain high amounts of hydrogen sulfide. Refineries processing such crude oil or natural gas commonly end up with substantial amounts of sulfide-containing scale on the metal surfaces in contact with the crude oil or gas. This scale is detrimental to the efficient operation of heat exchangers, cooling towers, reaction vessels, transmission pipelines, furnaces, etc. Removal of this sulfide-containing scale has been a substantial problem because conventional acidcleaning solutions react with the scale and produce gaseous hydrogen sulfide.
Hydrogen sulfide gas produced during the cleaning operation leads to several problems. First, hydrogen sulfide is an extremely toxic gas and previous techniques have required the entire system to be vented to an appropriate flare system (in which the gas is burned) or to a sodium hydroxide scrubbing system. Neither of these alternatives is very attractive because the sulfur dioxide and sulfur trioxide formed during the burning of hydrogen sulfide are substantial pollutants in and of themselves. The sodium sulfide produced during the scrubbing system is a solid that presents disposal problems. It can be land-filled or put into disposal ponds but only under conditions such that the sodium sulfide does not contact acid. Sodium sulfide reacts rapidly with acids to regenerate hydrogen sulfide. Second, aside from the toxic nature of hydrogen sulfide, the material causes operational problems as well because it is a gas. The volume of gas produced can be substantial. The gas takes up space within the unit being cleaned and can prevent the liquid cleaning solution from coming in contact with all of the metal surfaces. This can occur, for example, in cleaning a horizontal pipeline where the gas can form a "pad" over the top of the flowing liquid and prevent the liquid from filling the pipeline and cleaning the entire surface. The gas produced can also cause the pumps used in the system to cavitate, lose prime, and/or cease to function efficiently. And, of course, if enough gas is generated in a confined vessel the vessel can rupture.
These problems have been encountered in the industry and are severe.
Hydrogen sulfide and acid cleaning solutions containing hydrogen sulfide can cause severe corrosion problems on ferrous metals. The corrosion can be due to attack by acid and/or ferric ion corrosion. These corrosion problems have been met in the past by including minor amounts of corrosion inhibitors in the system. Aldehyde and aldehyde condensation products (normally with an amine) have been used as corrosion inhibitors in various systems. For example, they have been used alone or in combination with other corrosion inhibitors in aqueous acidic cleaning solutions and pickling baths or as an additive to crude oil. Under these systems, however, the aldehyde was included in very minor amounts. The following patents are representative of how these aldehydes have been previously used in this regard: U.S. Pat. No. 2,426,318; U.S. Pat. No. 2,606,873; U.S. Pat. No. 3,077,454; U.S. Pat. No. 3,514,410; and U.S. Pat. No. 3,669,613.
The reaction of hydrogen sulfide with an aldehyde is a known reaction which has been the subject of some academic interest. See, for example, the journal articles abstracted by Chemical Abstracts in C.A.54:17014h; C.A.63:14690a; C.A.65:9026d. The references indicate that the product formed by hydrogen sulfide with formaldehyde is trithiane or low polymers. This product was also referred to in U.S. Pat. No. 3,669,613 cited above. In these references, the product was produced by bubbling hydrogen sulfide through the aqueous acid/formaldehyde systems and the patent indicates that the reaction should not be attempted at temperatures greater than about 45.degree. C. The patent also indicates that the reaction usually reaches completion in from about 51/2 hours to about 91/2 hours at ambient temperatures.
None of the references taught or suggested the unique phenomenon observed by Frenier et al. and described in their U.S. patent application Ser. No. 967,047, filed Dec. 6, 1978, entitled "Composition and Method for Removing Sulfide-containing Scale From Metal Surfaces" (the disclosure of which is incorporated herein by reference). Frenier et al. discovered that acid-soluble, sulfidecontaining scale could be effectively removed from metal surfaces without the release of gaseous hydrogen sulfide by use of an aqueous acid cleaning composition comprising an aqueous non-oxidizing acid having at least one aldehyde dissolved or dispersed therein, with the aldehyde being present in such compositions in an amount at least sufficient to prevent or substantially prevent the evolution of hydrogen sulfide gas. This required at least a stoichiometric amount of aldehyde in the cleaning solution (i.e. at least one mole of aldehyde per mole of hydrogen sulfide produced during the cleaning) and an excess of aldehyde was preferred. By excess was meant, amounts beyond stoichiometric required and up to one equivalent weight of aldehyde or more per equivalent weight of acid. Their best system was an aqueous sulfuric acid cleaning solution containing excess formaldehyde.
The discovery by Frenier et al. represented a technical breakthrough in the chemical cleaning industry. Their discovery has been commercialized and widely accepted.